1. Field of the Invention
The present invention relates in general to a ceramic pressure sensor, and more particularly to a ceramic pressure sensor suitable for measuring the pressure within a cylinder of an internal combustion engine, or the pressures in similar environments.
2. Discussion of the Prior Art
There is known a pressure sensor of a type in which suitable strain detector means is provided on a diaphragm which is deformable in response to a pressure applied thereto. Deformation or strain of the diaphragm causes a change in an electrical output of the strain detector means, whereby the pressure applied to the diaphragm can be measured or determined by the output produced by the strain detector means. Recently, examples of this type of pressure sensor are proposed in SAE Reports 820319 and 860474.
In the known pressure sensor of the above-indicated type which employs a ceramic diaphragm, one of opposite major surfaces of the diaphragm directly communicates with the ambient space. Namely, that surface of the diaphragm is directly exposed to the atmosphere to be measured (hereinafter referred to as "measurement atmosphere").
When the pressure sensor is used for measuring the pressure (combustion pressure) within the cylinder bore of an internal combustion engine, for example, the diaphragm directly contacts a relatively hot gas produced in the combustion process of the engine, and a relatively cool gas existing in the intake process of the engine. Described in more detail, the diaphragm is subject to alternate heating and cooling by the combustion and intake gases at relatively short intervals. This causes a difference between the temperatures on opposite sides of the diaphragm, which induces a thermal stress that may result in a certain degree of deformation or deflection of the diaphragm.
When the diaphragm is exposed to the hot combustion gas, the above deflection of the diaphragm due to the thermal stress takes place in a direction opposite to the direction of the deflection of the diaphragm caused by a positive pressure of the measurement gas applied thereto. When the diaphragm is exposed to the relatively cool intake gas, the deflection due to the thermal stress occurs in the same direction as the direction of the deflection caused by the pressure. Accordingly, the pressure as detected by the strain detector means is lower than the actual pressure in the combustion process of the engine, and higher than the actual pressure in the intake process of the engine. Therefore, the pressure sensor suffers from a measuring error which corresponds to the amount of deflection of the diaphragm due to the thermal stress indicated above. Thus, the known pressure sensor is not sufficiently satisfactory in measuring accuracy.